Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density and high voltage have been commercialized and widely used.
A lithium metal oxide is used as a cathode active material of a lithium secondary battery, and lithium metal, a lithium alloy, crystalline or amorphous carbon, or a carbon composite is used as an anode active material. A current collector may be coated with the active material of appropriate thickness and length or the active material itself may be coated in the form of a film, and the resultant product is then wound or stacked with an insulating separator to prepare an electrode group. Thereafter, the electrode group is put into a can or a container similar thereto, and a secondary battery is then prepared by injecting an electrolyte solution.
Charge and discharge of the lithium secondary battery is performed while a process of intercalating and deintercalating lithium ions from a lithium metal oxide cathode into and out of a graphite anode is repeated. In this case, since lithium is highly reactive, lithium reacts with the carbon electrode to form Li2CO3, LiO, or LiOH. Thus, a film may be formed on the surface of the anode. The film is denoted as “solid electrolyte interface (SEI)” wherein the SEI formed at an initial stage of charging may prevent a reaction of the lithium ions with the carbon anode or other materials during charge and discharge. Also, the SEI may only pass the lithium ions by acting as an ion tunnel. The ion tunnel may prevent the destruction of a structure of the carbon anode due to the co-intercalation of the carbon anode and organic solvents of an electrolyte solution having a high molecular weight which solvates lithium ions and moves therewith.
Therefore, in order to improve high-temperature cycle characteristics and low-temperature output of the lithium secondary battery, a robust SEI must be formed on the anode of the lithium secondary battery. When the SEI is once formed during the first charge, the SEI may prevent the reaction of the lithium ions with the anode or other materials during repeated charge and discharge cycles caused by the subsequent use of the battery, and may act as an ion tunnel that only passes the lithium ions between the electrolyte solution and the anode.
In general, binary and tertiary electrolytes based on ethylene carbonate (EC) may be used as an electrolyte of a lithium-ion battery. However, since the EC has a high melting point, an operating temperature may be limited and battery performance may be significantly reduced at a low temperature.